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The Mi-9 Gene from Solanum arcanum Conferring Heat-Stable Resistance to Root-Knot Nematodes Is a Homolog of Mi-1^1,[W],[OA]

Department of Nematology, University of California, Riverside, California 92521

Resistance conferred by the Mi-1 gene from Solanum peruvianum is effective and widely used for limiting root-knot nematode (Meloidogyne spp.) yield loss in tomato (Solanum lycopersicum), but the resistance is ineffective at soil temperatures above 28°C. Previously, we mapped the heat-stable resistance gene Mi-9 in Solanum arcanum accession LA2157 to the short arm of chromosome 6, in a genetic interval as Mi-1 and the Cladosporium fulvum resistance gene Cf2. We developed a fine map of the Mi-9 region by resistance and marker screening of an F₂ population and derived F₃ families from resistant LA2157 x susceptible LA392. Mi-1 intron 1 flanking primers were designed to amplify intron 1 and fingerprint Mi-1 homologs. Using these primers, we identified seven Mi-1 homologs in the mapping parents. Cf-2 and Mi-1 homologs were mapped on chromosome 6 using a subset of the F₂. Cf-2 homologs did not segregate with Mi-9 resistance, but three Mi-1 homologs (RH1, RH2, and RH4) from LA2157 and one (SH1) from LA392 colocalized to the Mi-9 region. Reverse transcriptase-polymerase chain reaction analysis indicated that six Mi-1 homologs are expressed in LA2157 roots. We targeted transcripts of Mi-1 homologs for degradation with tobacco (Nicotiana tabacum) rattle virus (TRV)-based virus-induced gene silencing using Agrobacterium infiltration with a TRV-Mi construct. In most LA2157 plants infiltrated with the TRV-Mi construct, Mi-9-meditated heat-stable root-knot nematode resistance was compromised at 32°C, indicating that the heat-stable resistance is mediated by a homolog of Mi-1.

² Present address: Department of Botany and Plant Sciences, University of California, Riverside, CA 92521.

³ Present address: Arizona Genomics Institute, Department of Plant Sciences, University of Arizona, Tucson, AZ 85725.

⁴ Present address: Fundación AZTI-Tecnalia, Txatxarramendi ugartea, 48395 Sukarrieta, Spain.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Isgouhi Kaloshian (isgouhi.kaloshian{at}ucr.edu).

^[W] The online version of this article contains Web-only data.

^[OA] Open Access articles can be viewed online without a subscription.

www.plantphysiol.org/cgi/doi/10.1104/pp.106.089615

^* Corresponding author; e-mail isgouhi.kaloshian{at}ucr.edu; fax 951–827–3719.

Received September 7, 2006; accepted December 8, 2006; published December 15, 2006.

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